Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session B37: Focus Session: Graphene Stacking Sequence, Including Twisted Bilayers |
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Sponsoring Units: DMP Chair: Diego Mastrogiuseppe, Ohio University Room: 705/707 |
Monday, March 3, 2014 11:15AM - 11:27AM |
B37.00001: Synthesis of twisted bilayer graphene and studies of its low energy Raman modes Ting Fung Chung, Rui He, Conor Delaney, Courtney Keiser, Luis A. Jauregui, Paul M. Shand, C.C. Chancey, Yanan Wang, Jiming Bao, Yong P. Chen We have synthesized bilayer graphene on copper foils with different twist angles and stacking orders using chemical vapor deposition. Raman spectroscopy has been used to study twisted bilayer graphene (tBLG) transferred on Si/SiO2 substrate, focusing on low frequency Raman modes below 200 cm$^{-1}$. The modes are found in a small range of twist angle at which the G Raman peak is under resonance conditions with corresponding laser energy. The $\sim$ 94 cm$^{-1}$ mode (ZO'$_{\mathrm{L}})$ and $\sim$ 160 cm$^{-1}$ (ZO'$_{\mathrm{H}})$ modes (measured with a 532 nm laser) are assigned to the fundamental layer breathing vibration (ZO' mode) associated with different phonon wavenumbers, indicating different phonon scattering processes. We identify that the ZO'$_{\mathrm{L}}$ mode shares the same resonance enhancement mechanism as G Raman mode arising from van Hove singularities (vHs) in the band structure of tBLG. The ZO'$_{\mathrm{H}}$ mode was previously observed, related to the superlattice induced wavevector. The dependence of ZO'$_{\mathrm{L}}$ mode frequency and line width on the twist angle can be understood by the double-resonance Raman scattering. We also observe another lower energy Raman mode at $\sim$ 52 cm$^{-1}$, whose origin is yet to be understood. We have also measured the doping dependence of Raman modes in tBLG. Our results probe the interlayer coupling and phonon dispersions in tBLG. [Preview Abstract] |
Monday, March 3, 2014 11:27AM - 11:39AM |
B37.00002: Theory of twisted graphene bilayers near commensuration Hridis Pal, Steven Carter, Markus Kindermann It has been predicted [1,2] that a gap can arise in the Dirac spectrum of commensurately twisted graphene bilayers. Hitherto that gap has not been observed experimentally since it is difficult to produce samples with a specified twist angle. This motivates us to construct a long wavelength theory of almost commensurately rotated graphene bilayers. The theory inherits its structure from the exactly commensurate bilayer that it is close to. It thus makes the physics of commensurate graphene bilayers more easily accessible experimentally. [1] E. J. Mele, Phys. Rev. B 81, 161405 (R) (2010). [2] S. Shallcross, S. Sharma, and O. A. Pankratov, Phys. Rev. Lett. 101, 056803 (2008). [Preview Abstract] |
Monday, March 3, 2014 11:39AM - 11:51AM |
B37.00003: Twisted Bilayer Graphene with Controlled Rotation Angles Yanan Wang, Sirui Xing, Xiaoxiang Lu, Francisco Robles-Hernandez, Shin-shem Pei, Jiming Bao With unique rotation-angle dependent electronic band structure, twisted bilayer graphene (tBLG) is expected to be a promising platform for future semiconductor electronic and photonic applications. Although tBLG has been observed in the samples prepared by silicon sublimation of SiC, chemical vapor deposition (CVD), and stacking of single-layer graphene, tBLG with controlled rotation angles has not been demonstrated. In this work, we present a simple transfer method to create tBLG domains with pre-defined rotation angles in the size of a few hundred micrometers. [Preview Abstract] |
Monday, March 3, 2014 11:51AM - 12:27PM |
B37.00004: Theoretical studies of structure-property relations in graphene-based carbon nanostructures Invited Speaker: Dimitrios Maroudas This presentation focuses on establishing relations between atomic structure, electronic structure, and properties in graphene-based carbon nanostructures through first-principles density functional theory calculations and molecular-dynamics simulations. We have analyzed carbon nanostructure formation from twisted bilayer graphene, upon creation of interlayer covalent C-C bonds due to patterned hydrogenation or fluorination. For small twist angles and twist angles near 30 degrees, interlayer covalent bonding generates superlattices of diamond-like nanocrystals and of fullerene-like configurations, respectively, embedded within the graphene layers. The electronic band gaps of these superlattices can be tuned through selective chemical functionalization and creation of interlayer bonds, and range from a few meV to over 1.2 eV. The mechanical properties of these superstructures also can be precisely tuned by controlling the extent of chemical functionalization. Importantly, the shear modulus is shown to increase monotonically with the fraction of \textit{sp}$^{\mathrm{3}}$-hybridized C-C bonds. We have also studied collective interactions of multiple defects such as random distributions of vacancies in single-layer graphene (SLG). We find that a crystalline-to-amorphous structural transition occurs at vacancy concentrations of 5-10{\%} over a broad temperature range. The structure of our defect-induced amorphized graphene is in excellent agreement with experimental observations of SLG exposed to a high electron irradiation dose. Simulations of tensile tests on these irradiated graphene sheets identify trends for the ultimate tensile strength, failure strain, and toughness as a function of vacancy concentration. The vacancy-induced amorphization transition is accompanied by a brittle-to-ductile transition in the failure response of irradiated graphene sheets and even heavily damaged samples exhibit tensile strengths near 30 GPa, in significant excess of those typical of engineering materials. [Preview Abstract] |
Monday, March 3, 2014 12:27PM - 12:39PM |
B37.00005: Scaling laws of van Hove singularities in twisted bilayer graphene Jeil Jung, Ashley DaSilva, Yang Wang, Dillon Wong, Michael Crommie, Shaffique Adam, Allan H. MacDonald Van Hove singularities (vHS) appear in twisted coupled bilayer graphene at saddle points in the band structure. The lowest energy vHS can be associated with the overlap between the displaced Dirac cones of the top and bottom layers, resulting in an approximately linear increase of its position in energy with increasing twist angle. This picture, which is applicable in the perturbative regime for moderately large twist angles, sees departures in the small angle limit due to non-perturbative coupling between the layers. Using a theory for twisted bilayer graphene [1] that incorporates all the relevant interlayer coupling compatible with momentum conservation of k-vectors of the top and bottom layers we explore the scaling laws of the vHS for sufficiently small twist angles and long period moire superlattices. We analyze the localization properties of their wave functions through their local density of states (LDOS) paying particular attention to the behavior of the states corresponding to higher energy van Hove singularities. We comment on our results in light of the experimental DOS and LDOS maps obtained through scanning tunneling microscopy. [1] R. Bistritzer and A. H. MacDonald, PNAS 108 (30), 12233 (2011) [Preview Abstract] |
Monday, March 3, 2014 12:39PM - 12:51PM |
B37.00006: First-Principles Calculations of Off-Normal LEEM Reflectivity Spectra of Few Layer Graphene John McClain, Karsten Pohl, Jian-Ming Tang We present calculations of the off-normal low-energy electron specular reflectivity spectra of few layer graphene (FLG) systems using our first-principles theoretical approach that leverages the self-consistent scattering potentials produced by density-functional theory [1]. Our Bloch wave matching approach, which replaces the traditional analysis using multiple scattering off muffin-tin potentials, admits a straightforward handling of non-normal incident beams. Our calculated off-normal spectra for free-standing FLG reveal the shifting of the characteristic thickness-dependent oscillations in reflectivity found for energies between 0 and 7 eV in normal-incidence low-energy electron microscopy (LEEM) spectra. We also find shifts in other peaks and new features for incoming beams with in-plane momentum far from $\Gamma$. We compare the spectra to features in the in-plane band structure of FLG and to available experimental LEEM and LEED data for FLG on metallic and semiconductor substrates. We discuss modeling reflection for small deviations from normal incidence, as well as the possibility of accessing novel spectra features using wide-angle scattering. [1] McClain et al., arXiv.1311.2917. [Preview Abstract] |
Monday, March 3, 2014 12:51PM - 1:03PM |
B37.00007: Electrochemical Intercalation of Few-Layer Graphenes: Method and Characterization Shu Yang Frank Zhao, Giselle A. Elbaz, Dmitri K. Efetov, Jayakanth Ravichandran, Yinsheng Guo, Natalee Raymond, Louis Brus, Xavier Roy, Philip Kim Few layer graphene (FLG) intercalate compounds form a new generation of graphene derivative systems where novel physical phenomena such as superconductivity and magnetism may emerge. Experimental realization of FLG has been limited to the harsh intercalation processes which are often not compatible with mesoscopic device fabrication techniques. We demonstrate the in-situ intercalation and transport measurements of mechanically exfoliated FLGs using alkali metals via electrochemical methods. With suitable passivation methods, we isolate the FLG's contribution to the electrochemical current, and electronically monitor the intercalation reaction in real time, via cyclic voltammetry. We correlate the intercalation signatures from cyclic voltammetry with optical and Raman characteristics of the FLGs. Finally, we characterize the intercalated few-layer graphene compounds by transport measurements down to cryogenic temperatures. [Preview Abstract] |
Monday, March 3, 2014 1:03PM - 1:15PM |
B37.00008: Exciton Effects on Doped and Gated Twisted Bilayer Graphene Ryan Soklaski, Yufeng Liang, Li Yang Turbostratic graphite and epitaxially grown few-layer graphene (FLG) are known to exhibit significant rotational defects -- a departure from familiar Bernal stacked FLG systems. The admixing of states across rotated graphene layers occur far from their respective valleys, giving rise to saddle-point van Hove singularities. We study the effects of doping and voltage gating on twisted bilayer graphene. In particular, we perform first-principle calculations, including e-e and e-h interactions, of the optical absorption spectra of doped and gated twisted bilayer graphene. Increasing the doped carrier density enhances screening in the system, reducing both the self-energy corrections and e-h interaction effects -- an effect also seen in doped single layer graphene. On the other hand, gating the system leads to a misalignment of van Hove singularities, diminishing the joint density of states and hence the exciton strength. [Preview Abstract] |
Monday, March 3, 2014 1:15PM - 1:27PM |
B37.00009: Optical properties of twisted bilayer graphene Pilkyung Moon, Young-Woo Son, Mikito Koshino A twisted stack of two graphene layers (twisted bilayer graphene) exhibits an extremely long potential period arising from the Moir\'{e} interference between the layers. We calculate the optical absorption of twisted bilayer graphene in the absence of magnetic field and demonstrate that the spectroscopic characteristics serve as a fingerprint to identify the rotation angle between two layers\footnote{P. Moon and M. Koshino, Phys.\ Rev.\ B \textbf{87}, 205404 (2013).}. We explain the peculiar optical selection rule in terms of the symmetry of the effective Hamiltonian. We also investigate the effects of charging and gating on the optical spectrum\footnote{P. Moon, M. Koshino, and Y.-W. Son, in preparation}. In addition, we investigate the absorption spectrum and the selection rule for the fractal band regime (Hofstadter butterfly) in the presence of magnetic field. We demonstrate that the absorption spectrum exhibits a self-similar recursive pattern reflecting the fractal nature of the energy spectrum, and the optical selection rule has a nested self-similar structure as well\footnote{P. Moon and M. Koshino, arXiv:1308.0713 (2013)}. [Preview Abstract] |
Monday, March 3, 2014 1:27PM - 1:39PM |
B37.00010: Incoherent interlayer conduction in twisted bilayer graphene Youngwook Kim, S.-G. Nam, H.-J. Lee, Jun Sung Kim, H. Yun, S.W. Lee, M. Son, H.C. Choi, D.S. Lee, D.C. Kim, S. Seo Coherent motion of the electrons in the Bloch states often breaks down for the interlayer conduction in layered materials where the interlayer coupling is significantly reduced by e.g. large interlayer separation. Here, we report complete suppression of coherent conduction in twisted bilayer graphene even with an atomic length scale of layer separation. The interlayer conduction were investigated using a cross junction of monolayer graphene layers. The interlayer resistivity is much higher than the c-axis resistivity of Bernal-stacked graphite and exhibits strong dependence on temperature as well as on external electric fields. These results suggest that the graphene layers are significantly decoupled by rotation, and the incoherent electron tunneling is the main interlayer conduction channel. In this regime, the interlayer conduction is determined by the overlap of the Dirac Fermi surfaces (FS) from each layer. The angle dependence of the interlayer resistivity is found to be relatively strong at low temperatures, while it becomes moderate and monotonous at high temperatures. This demonstrates the importance of phonon-mediated conduction at high temperatures, which enhances the overlap between the momentum-mismatched FS's in twisted bilayer graphene. [Preview Abstract] |
Monday, March 3, 2014 1:39PM - 1:51PM |
B37.00011: Gated Raman Spectroscopy of Twisted Bilayer Graphene Shengqiang Huang, Kanokporn Chattrakun, Matthew Yankowitz, Arvinder Sandhu, Brian LeRoy The interaction of charge carriers with lattice vibrations in graphene exhibits many intriguing physical phenomena. Raman spectroscopy is a powerful non-destructive technique to probe these interactions. In twisted bilayer graphene, the electronic band structure and phonon dispersion depend on the rotation angle between the layers. Here we present a systematic Raman spectroscopy study of twisted bilayer graphene, using a 532 nm laser, with controllable charge densities up to 2$\times$10$^{13}$cm$^{-2}$. The twist angle is first identified by the observation of a moire pattern in STM measurements. In the angle range between 5 and 8 degrees, the R$'$ peak softens and weakens with increasing charge density. Near 12 degrees, the G peak is enhanced due to the increased density of states in twisted bilayer graphene. However, the G peak area quickly decreases with increasing charge density. Lastly, we observed several unusual effects for the G peak for all angles from 2 to 10 degrees as a function of increasing charge density. We found that the G peak broadened, split and oscillated in position. All these results demonstrate that twisted bilayer graphene has rich optoelectronic properties. [Preview Abstract] |
Monday, March 3, 2014 1:51PM - 2:03PM |
B37.00012: A Scanning Tunneling Study of Twisted Bilayer Graphene Dillon Wong, Yang Wang, Jeil Jung, Ashley DaSilva, Sergio Pezzini, Hsin-zon Tsai, Han Sae Jung, Ramin Khajeh, Youngkyou Kim, Salman Kahn, Sajjad Tollabimazraehno, Haider Rasool, Juwon Lee, Kenji Watanabe, Takashi Taniguchi, Alex Zettl, Shaffique Adam, Allan MacDonald, Michael Crommie The properties of bilayer graphene strongly depend on the angle of rotation between its two layers. We investigated the local electronic structure of twisted bilayer graphene on an insulating substrate. Using scanning tunneling microscopy, we measured the energy dependence of features in the differential tunneling conductance for many different twist angles. Comparison with theoretical calculations reveal the physical origin of these features. [Preview Abstract] |
Monday, March 3, 2014 2:03PM - 2:15PM |
B37.00013: Exchange self-energy and compressibility of multilayer graphene by wavefunction rotation method Hongki Min Multilayer graphene has chiral electronic structure which strongly depends on the stacking sequence. A fundamental issue is to understand the interplay between the chiral electronic structure and electron-electron interaction, and the exchange interaction is the leading-order correction to the electron-electron interaction. The exchange energy calculation of multilayer graphene, however, requires a large amount of computational cost, because of non-local nature of the exchange interaction and because of the absence of the analytic form of the wavefunction, which should be obtained self-consistently. We overcome this problem using the wavefunction rotation method, in which the angular part of the wavefunction is obtained analytically by attaching a phase factor that is determined by the stacking sequence, thus reducing the computational cost significantly. Using this method, we calculate the exchange self-energy and compressibility of multilayer graphene, and discuss the role of intralayer and interlayer exchanges. [Preview Abstract] |
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